1. The Field of the Invention
This invention relates to diagnostic systems used in medical applications. More particularly, the present invention relates to a pressure multiplexing system which is particularly adapted to allow two hemodynamic pressures to be alternately measured using only a single pressure transducer.
2. The Background Art
The capability of modern medicine to extend the life of critically ill patients and reduce the risks associated with major surgery is greatly dependent upon the use of sophisticated medical equipment and procedures. Such medical equipment includes various diagnostic devices used for monitoring a patient's biological functions. One indicator of biological function is the pressure of various fluids within the body. For example, blood pressure has long been used as one measure of a patient's condition.
Indirect blood pressure measurement may be obtained at a peripheral artery using a pressure cuff and manometer. However, in critically ill patients or patients undergoing major surgery, and particularly in patients who are being treated for coronary disease, it is extremely useful for the attending physician to be able to monitor blood pressure within the arteries and veins adjacent to the heart and even within the various chambers of the heart. It is possible to measure such pressures by using a catheter inserted into the body.
A common cardiac catheterization procedure requires the use of a pulmonary artery flow directed catheter. This type of catheter is also commonly referred to as a "Swanz-Ganz" catheter after the developers of the catheter and the corresponding procedure. The Swanz-Ganz catheter is a multilumen catheter which is also equipped with a thermistor to allow measurement of cardiac output by the thermodilution method.
The catheter is generally used by inserting it into a vein in the neck, arm, or leg where it is moved along the vein to the proper position in the heart. The catheter is referred to as a "flow directed" catheter because it passes through the right atrium into the right ventricle where a small balloon located at the tip of the catheter is inflated and the flow of blood out of the right ventricle directs the tip of the catheter into position in the pulmonary artery. Once in place, one lumen of the catheter opens into the right atrium. The pressure sensed in the right atrium of the heart is referred to as the central venous pressure, which will hereinafter be referred to as the "CVP."
Another lumen of the catheter opens near the tip of the catheter which is positioned in the pulmonary artery and is used to sense the pulmonary artery pressure or the pulmonary artery wedge pressure, which will hereinafter collectively be referred to as "PAP."
Once the catheter is properly positioned, and any bubbles are flushed from the lumens, the pressure within the right atrium and pulmonary artery is directly transmitted to the external end of the catheter by a column of fluid contained within the lumens of the catheter. The pressures communicated by the lumens may be directly measured by connecting each of the proximal ends of the lumens to a transducer (capable of converting the fluidic pressure into a proportional electric signal) and a monitor (a device used to display and/or record the value of the pressure detected by the transducer).
Transducers and monitors are relatively expensive devices which may often be in short supply in a hospital. Thus, when two or more pressures are to be monitored, it has been a common practice to monitor the most critical pressures using a transducer and monitor setup (the most accurate technique) and the less important pressures are monitored using a manometer. In heart catheterization procedures, it is common to continually monitor the PAP while the CVP is only of intermittent interest and thus often is monitored using a water manometer.
Using the transducer to measure one pressure and a water manometer to measure another pressure has many undesirable attributes. Included in these undesirable attributes is the inherent inaccuracy of a manometer as well as the fact that an error is easily introduced when reading the manometer or by the fact that two different transducers which have different calibration are used which could result in offset or gain errors. Furthermore, errors can be introduced when converting a reading taken in millimeters of water to a more familiar scale such as millimeters of mercury. Most electronic monitors are calibrated to provide readings in millimeters of mercury. Furthermore, a water manometer is bulky and fragile.
In the case of monitoring PAP and CVP, many medical professionals have adopted the practice of alternately monitoring the PAP and CVP using a single transducer and a monitor (or a single channel on a monitor). In this way, the PAP may be monitored nearly continuously while the CVP may be monitored intermittently, and the disadvantages of using a water manometer are eliminated.
In order to allow a single transducer to monitor two or more pressures (such as PAP and CVP), several techniques have been devised. All of the techniques previously available make use of a valve assembly common to the medical industry and referred to as a "three-way stopcock."
Such stopcock assemblies, while being relatively inexpensive, have many drawbacks. For example, multiple stopcock assemblies require additional time to set up and also require additional time to switch the transducer between the two or more sources of pressure. Perhaps the greatest disadvantage of multiple stopcock assemblies is that they are often confusing to those operating them and may cause an operator to erroneously ser the stopcocks, potentially causing fatal injury to the patient.
Injury to the patient is particularly a concern in view of the fact that medications are commonly administered (sometimes at relatively high rates) by way of the catheter. It will be appreciated that incorrect operation of multiple stopcock assemblies could result in the infusion of a drug into an incorrect location with disastrous results for the patient. For example, it is sometimes desirable to infuse drugs into the right atrium of the heart (the location of the CV lumen) at a rate which would cause severe difficulties if infused at the same rate directly into the pulmonary artery (the location of the PA lumen).
Furthermore, applying high infusion rate pressures directly to a transducer, which may occur in some multiple stopcock assemblies, may damage the transducer. Still further, multiple stopcock assemblies are often cumbersome to mount on a bedside pole and may be easily damaged.
Also, multiple stopcock assemblies may inadvertently become disassembled during use. Of course, when multiple stopcocks are being used, the incorrect setting of any one stopcock, if not causing more serious consequences (such as catheter clotting off due to lack of continuous infusion of fluid), will likely cause an incorrect pressure reading to occur.